In the seminiferous tubules of rat testis we found a Mn 2 ion-sensitive, Mg 2 ion- and fluoride-insensitive adenyl cyclase developing during speriogenesis. An adenyl cyclase of similar properties is present in epididymal spermatozoa. The stimulatory effect of Mn 2 ion on the tubular (presumably spermatid cell) and spermatozoal adenyl cyclase is augmented by Ca 2 ion. While in the seminiferous tubules the adenyl cyclase appears to be in the cytosol, in the sperm the cyclase system is firmly associated with membranes. We will attempt to establish the specific cell type where this distinctive adenyl cyclase is first synthesized, and the developmental stage at which it becomes attached to the membranes. In our previous studies we found that the adenyl cyclase developing during speriogenesis is remarkably soluble and extremely stable in contrast to most adenyl cyclases in a variety of mammalian somatic cell types. Taking advantage of these properties of the tubular adenyl cyclase we propose the chemical purification and characterization of this adenyl cyclase system. The molecular nature and the mode of activation of the germ-cell adenyl cyclase will be pursued.